handle relations in show mode

fix #306

porting the configuration for show templates from the orm admin.
the [type guesser](https://github.com/sonata-project/SonataDoctrinePhpcrAdminBundle/blob/master/Guesser/TypeGuesser.php#L62-L75) handles some more things. not sure how to correctly handle these here - ideally we would have a central method to check if something is an association. the phpcr-odm class metadata class could tell, but not sure if its available here.